Plasma reactor

ABSTRACT

The invention relates to a plasma reactor including a housing which receives a selected gas flow, a microwave generator, and waveguide means to supply the microwaves to the housing according to a non-resonant coupling wherein the housing does not dissipate in the form of electromagnetic radition the ultrahigh frequence energy which is imparted to the housing in the presence of gas. According to a general definition of the invention, the housing being of a large size, the terminal portion of the waveguide means is progressively reduced in one direction and widened in the other direction till it reaches a flat rectangular cross-section surrounding completely the housing, and it forms a non-resonant coupling with the housing.

BACKGROUND OF THE INVENTION

The invention relates to plasma reactors. It finds an application in thesurface treatment of samples, in particular, in micro-electronics.

Plasma reactors are already known that comprise a housing placed in avacuum capable of receiving a selected gas flow, a microwave generatorand wave guide means for passing the microwaves to the housing.

To render the plasma homogeneous with a view to obtaining a bettersurface treatment of samples for example, provision is most frequentlymade for applying a magnetic diffusion field in the housing.

Now the application of such a magnetic diffusion field has the drawbackthat it consumes a large amount of energy, sometimes exceeding that usedto produce the plasma.

The result is a limitation of the reactor dimensions and therefore ofthe dimensions of the plasma obtained, which prevents the treatment ofsamples with large diameters.

SUMMARY OF THE INVENTION

The object of the invention is to provide a solution of this problem.

More particularly, it aims to improve the flexibility of use of theplasma reactor and its versatility in application.

The invention relates to a plasma reactor comprising: a housing capableof receiving a selected gaseous flow, a microwave generator and waveguide means for passing the microwave to the housing along anon-resonant coupling wherein the housing does not dissipate, in theform of electromagnetic radiation, the ultrahigh frequency energyimparted thereto in the presence of gas.

According to a general definition of the invention, since the housing isof a large size, the end portion of the wave guide means isprogressively reduced in one direction and widened in the other, untilit reaches a flat rectangular cross section completely surrounding thesaid housing and produces a non-resonant coupling therewith.

According to one aspect of the invention, provision is made for a pistontype ultrahigh frequency short circuit means, completely surrounded bythe end portion of the wave guide means and disposed on thediametrically opposite side relative to the microwave intake, the saidpiston being adjusted so as to define a desired ultrahigh frequencyelectric field in the housing.

In the field of industrial applications of the plasma reactor inaccordance with the invention, the ultrahigh frequency short circuitmeans is advantageously immovable.

In the field of experimental applications of the plasma reactor inaccordance with the invention, the ultrahigh frequency short circuitmeans is preferably movable.

According to a preferred mode of embodiment of the invention, thehousing comprises an internal tubular element placed in a vacuum andbeing concentric with the housing, the wall of the said tubular elementbeing constituted by a material having low dielectric losses, such asquartz.

According to another characteristic of the invention, provision is madeinside the housing for an additional ultrahigh frequency short circuitmeans coupled with the tubular element and having a first and secondannular disc surrounding the said tubular element, the annular discsbeing separated from each other by a predetermined and adjustabledistance with a view to confining the plasma in the portion of thetubular element comprised between the said annular discs.

In the field of industrial applications of the plasma reactor inaccordance with the invention, the additional ultrahigh frequency shortcircuit means is immovable.

In the field of experimental applications of the plasma reactor inaccordance with the invention, the additional ultrahigh frequency shortcircuit means is movable.

According to another aspect of the invention, the wave guide comprisesin its intermediate portion, adjustment means such as a plurality ofadjustable penetrating screws.

As a variant, the wave guide comprises in its intermediate end portiontapering in one direction and widened in the other, adjustment meanssuch a plurality of adjustable penetrating screws.

In the sphere of a plasma reactor intended for the surface treatment ofa sample, the sample rests on a sample carrier placed inside the tubularelement and in the portion of the tubular element comprised between theannular discs of the additional ultrahigh frequency short circuit means.

According to another preferred mode of embodiment of the invention, thesample carrier is placed into the tubular element and outside theportion comprised between the annular discs of the additional ultrahighfrequency short circuit means.

In the field of applications to the engraving of substrates and/or resinremoval, the plasma reactor comprises moreover means for generating anelectromagnetic field between the delivery means of the gaseous flow andthe sample carrier with a view to polarizing the sample.

Advantageously the plasma reactor comprises, moreover, heating means forthe heating of the samples.

According to another characteristic of the invention, provision is madefor a lock chamber for the insertion of the samples to allow the sampleto be placed on the sample carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent on examining the detailed description given below, as well asthe drawings wherein:

FIG. 1 is a schematic cross sectional view of the plasma reactor inaccordance with the invention;

FIG. 2 is a top view of the plasma reactor of FIG. 1;

FIG. 3 is a schematic general view of the main means that constitute theplasma reactor in accordance with the invention; and

FIG. 4 is a cross sectional view of the plasma reactor in accordancewith the invention provided with means for polarizing the sample.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The attached drawings comprise many elements of a definite nature whichcan only be provided by the drawings. They therefore form an integralpart of the description to help with the understanding of the invention,as well as to define it.

A plasma reactor (FIGS. 1 and 2) comprises a metallic housing (10) madefor example, of copper having a generally cylindrical shell.

The housing 10 comprises an inner cylindrical tube 12, concentric withthe housing 10 and whose wall is constituted by a material with lowdielectric losses, such as quartz.

The tube 12 has at each of its ends 14 and 16, a circular opening.

The lower portion of the housing 10 is provided with a circular opening18 concentric with the housing 10 with a diameter substantially equal tothat of the opening 16 of the tube 12. The circular openings 16 and 18are superposed so that the tube 12 communicates with the outside of thehousing 10.

The upper portion of the housing 10 is also provided with a circularopening 20 concentric with the housing 10, with a diameter substantiallyequal to that of the opening 14 of the tube 12. The circular opening 20is completely covered by a metallic circular cover 22, made of aluminiumfor example. The cover 22 is pierced at its center by an openingallowing a tube 24 to pass, whose end 26 issues in the tube 12.

Reference will now be made to FIG. 3. A selected gaseous flow circulatesin the tube 24. The tube 24 may be connected by conventional means tobottles 28 containing gases such as argon, oxygen, helium etc. . . .

Control means 30 such as flow meters and flow controllers allow the flowof the gaseous flow in the tube 24 to be controlled, as well as toobtain a selected gas mixture.

Reference is again made to FIGS. 1 and 2. The plasma reactor alsocomprises a microwave generator 32 capable of operating at a frequencyof the order of 2.45 gigahertz and at a power of 1.2 kilowatt.

Wave guide means 34 pass the microwaves to the housing 10 along anon-resonant coupling wherein the housing 10 does not dissipate, in theform of electromagnetic radiation, the ultrahigh frequency energyimparted to it in the said housing 10 in the presence of gas. Theinitial portion 36 and intermediate portion 37 of the wave guide meanshave a rectangular cross section. The electromagnetic energy ispropagated in this guide in the longitudinal direction of the guide 34(arrow a), the electric field E being orientated transversely to thedirection a.

In the initial portion 36 of the wave guide 34, provision is made for abidirectional coupler 39. In their intermediate portion 37, the waveguide means comprise a plurality of penetrating adjustment screws 40(for example, three in number) allowing the impedance of the reactor tobe adjusted so as to obtain a good output of the ultrahigh frequencytransmission towards the housing 10.

As a variant, the screws 40 may be disposed in the end portion 38 of thewave guide means which will be described below.

According to the main characteristic of the invention, the end portion38 of the wave guide means is progressively tapered in one direction(the vertical direction), and widened in the other (the horizontaldirection) until it attains a flat rectangular cross section completelysurrounding the housing 10.

In its intermediate portion, the housing 10 is subdivided into two halfhousings 11 and 13 separated from each other by a space (gap orclearance) with a rectangular cross section substantially equal to thatof the end portion 38 of the wave guide means so as to be completelysurrounded by the said end portion 38 of the wave guides. Thus the endportion 38 of the wave guides completely surround the housing 10 in theregion of the separation of the two half shells 11 and 13 so as to passthe microwaves to the tube 12.

The rectangular cross section of the end portion 38 of the wave guidesis greater than that of the space separating the two half shells 11 and13. Thus a space (gap or clearance) is arranged which permits anon-resonant coupling of the wave guide to the housing 10.

The original geometrical shape of the end portion of the wave guide,that is to say, tapering in one direction and widening in the other,allows the electromagnetic energy to be evenly distributed over thewhole cross section of the housing 10. The result is a much greaterexcitation zone than that proposed by the conventional cylindrical orrectangular wave guides. Moreover, the end portion 38 is constricted atits end so as to increase the directivity of the incidentelectromagnetic waves.

In theory, such an end portion of the wave guide should produce waves ofa higher mode in the housing, liable to dissipate the ultrahighfrequency energy imparted to the housing via the wave guide.

Now, the applicant has found, surprisingly, that such a plasma reactorcomprising a wave guide having a flat rectangular cross sectioncompletely surrounding the housing constitutes an absorbent structurewherein the higher mode waves are not generated.

On the diametrically opposite side in relation to the microwave intake34, provision is made for a non-resonant coupling to the housing alsocompletely surrounded by the end portion of the wave guide 38.

The non-resonant coupling is a piston-type short circuit means 50. Thepiston 50 comprises a flat, substantially rectangular cross sectionequal to that of the end portion of the wave guide means 38.

The coupling is non-resonant, because in the presence of plasma in thetube 12, the coupling does not dissipate, in the form of electromagneticradiation, the ultrahigh frequency energy imparted thereto.

The position of the piston 50 is adjusted so as to define a desiredultrahigh frequency electrical field in the housing 10. It will beobserved that the short circuit piston 50 has the same electromagneticwave distribution characteristics as the end portion of the wave guides38, which makes it possible to ensure symmetry in the propagation of thewaves. The adjustment of the position of the piston 50 allows couplingsbetter than 90% to be obtained. As for the rectangular wave guide means34 provided with its adjustment screws 40, it allows the coupling to beimproved to over 95%.

In the field of industrial applications, the position of the piston 50is fixed, whilst in the field of experimental applications, the piston50 is movable.

Within the housing 10, provision is made for a second ultra-highfrequency short circuit means 52 coupled to the tube 12. This secondshort circuit means 52 comprises two annular discs 53 and 54 that arealso of the piston type.

The plasma is generated in the tube 12 and can be confined to a greateror lesser extent by means of the two annular discs 53 and 54 whoseheight is adjustable.

The pistons 53 and 54 are thus interspaced from each other by apredefined and adjustable distance with a view to confining the plasmain the portion of the tube 12 comprised between the two pistons 53 and54.

In the field of industrial applications, the pistons 53 and 54 arefixed, whilst in the field of experimental applications, the pistons 53and 54 are movable.

Reference is again made to FIG. 3.

The circular opening 18 of the lower portion of the housing 10communicates with the outside via another cylindrical opening 60 with adiameter substantially equal to that of the opening 18 and formed in thecover of a metallic box 61, of aluminum for example, whereon the housing10 is supported. The openings 16, 18 and 60 are superposed, so that thetube 12 should communicate with the inside of the box 61.

The box 65 is of a general rectangular shape, having on one of itslateral faces a lock chamber 62 coupled to a slide type valve, to allowthe samples to be inserted into the tube where the plasma is generated.

A sample 67 intended to be treated by the plasma is passed from theoutside of the box 61 to a sample carrier 66 by means of a transfer tube65. The sample carrier 66 is placed inside the box 61 and concentricallywith the tube 12. This sample carrier 66 is movable in verticaltranslation, which allows it to carry the sample 67 outside or in theplasma being generated in the tube 12. In the case of a surfacetreatment of samples comprising fragile materials such as polymers, itis preferable for the sample to be carried outside the plasma and closethereto.

The tube 12 and the box 61 are evacuated by primary pumping means 70,establishing a weak vacuum and by secondary pumping means establishing astronger vacuum. The secondary pumping means 72 are ROOTS type pumpshaving a delivery of the order of 250 cubic meters per hour.

A pressure gauge 74 completes the vacuum appliance. This gauge 74 has aninput connected to the box 61 and an output connected to a pressurecontroller 76. In response to the pressure measured by the gauge 74, thepressure controller 76 actuates a motor (not shown) actuating theopening/closing of a valve 78 connected to the secondary pumping means72 with a view to controlling their delivery.

Reference will now be made to FIG. 4. For the treatment of particularsurfaces, in particular for the engraving of a silicon oxide layerdeposited on a substrate in microelectronics, it has been establishedthat a microwave plasma on its own does not allow a quick engraving ofthe silicon oxide to be obtained.

In this case, the plasma reactor in accordance with the inventioncomprises, moreover, generating means 80 capable of superposing anelectromagnetic radio frequency field between the means delivering thegaseous flow 24 and the sample carrier 66 with a view to increasing thespeed of engraving.

The generating means 80 deliver, for example, an AC voltage with afrequency of the order of 13 megahertz. This AC voltage is applied, forexample, to the sample carrier 66, whilst the housing 10 is connected toearth 82.

The applicants have obtained engraving speeds of several thousands ofangstroms per minute with an engraving quality of good homogeneity. Thisresult is explained by the fact that the microwave plasma is very richin positive ions. In fact, these positive ions bombard the negativelypolarized sample 67 in the presence of the electromagnetic fieldgenerated by the application of the AC voltage to the sample carrier 66.

For surface treatments of samples such as the removal of resin, theapplicants have obtained a homogeneous attack on the resin of the orderof several microns per minute. The best results as regards thehomogeneity of attack were obtained by a substrate placed outside theplasma. The gas used for the resin removal was pure oxygen or an oxygenbased mixture.

Sometimes, it is also necessary to heat the sample. In that case, theplasma reactor comprises, moreover, heating means capable of heating thesample. Advantageously, the electromagnetic field generated byapplication of the AC voltage allows the sample 67 to be heated.

The plasma reactor is operated as follows:

first, the coupling between the piston 50 and the wave guide 34 isadjusted as described above;

then the additional short circuit means 52 is adjusted so as to confinethe plasma between the two annular discs 53 and 54 of this short circuitmeans 52 with the tube 12.

Experiments have then been undertaken with filling pressures for thereactor, variable between 10 milli-torr and 10⁴ milli-torr for thefollowing gaseous configurations: pure oxygen, pure argon and anargon-oxygen mixture, wherein the argon percentage by volume amounted to66%. The microwave power applied was 800 watts. The adjustment of theshort circuit means 50 and of the screws 40 was effected so as to give aminimum reflected power at approximately 10² milli-torr.

As regards pure oxygen, it was then possible to observe that the plasmareactor showed a remarkable stability with a reflected power/total powerratio, also called "TOS", for "Taux d'Ondes Stationnaires"--Rate ofStationary Waves--which did not exceed 2% between 10 milli-torr and 1torr. In excess of one torr, these "TOS" rates increased very rapidlyuntil the plasma was extinguished.

With pure argon, the "TOS" rates remained low (less than 7%) butnevertheless, clearly higher than with pure oxygen.

With an argon-oxygen mixture, the pressure operating range extendedbeyond one torr, with "TOS" rates of the order of a few percent. Withthis argon oxygen mixture, there was again found on the one hand, thewide operating range with argon and on the other hand, a plasma whichmade it possible to produce high pressure reactive oxygen.

Other tests have been undertaken with the following experimentalconditions. The applied microwave power could be varied between 200 and1200 watts.

It could then be observed that the operation of the plasma reactorremained stable with "TOS" rates of less than 7%, and with virtuallyperfect adjustment regions. On the other hand, and in conformity withthe earlier results, the stability of the reactor is better with pureoxygen than with argon or with an argon-oxygen mixture.

These tests show that the plasma reactors in accordance with theinvention have the advantage of a large operating range both as regardspressure and power with low, and even very low "TOS" rates.

Moreover, a very stable plasma can be obtained with relative variationsin pressure and/or high power. The conditions for obtaining the plasmaare easily reproducible. Other gases, such as ethylene, nitrogen,silicon chloride may be used for the gaseous flow, while retaining thesame operating characteristics of the reactor.

The applicants have also observed that the distribution of theelectromagnetic field has its maximum near the wall of the tube 12 andits minimum at the center of the said tube. In these conditions, itseems that the ultrahigh frequency waves produced in the gas areessentially surface waves. The plasma which is thus homogeneous in theportion comprised between the annular discs 53 and 54 of the additionalshort circuit means 52 is then the locus of stationary ultrahighfrequency waves of very high intensity.

The applicants have also observed that the excitation frequency, that isto say, the frequency of the microwave generator affects the electronicdensity and the volume of the plasma.

They have also observed that when the excitation frequency is reduced,the electronic density is reduced as a corollary, but at the same time,it is possible to increase the volume of the plasma. Thus, for theapplications requiring, for example, a plasma of a very great volume anda low electronic density, it is necessary to reduce the excitationfrequency and vice versa.

The various tests were made with a plasma reactor whose dimensions areas follows.

The dimensions of the housing 10 were: diameter approximately 400millimeters, height 300 millimeters.

The dimensions of the tube 12 were: diameter approximately 195millimeters, height approximately 300 millimeters.

In its intermediate portion 37, the dimensions of the wave guide were43×86 millimeters in cross section.

The dimensions of the short circuit means 50 were approximately 400×10millimeters in cross section.

In its end portion, the dimensions of the wave guide were approximately400×10 millimeters in cross section.

The advantages of the plasma reactor in accordance with the inventionare as follows:

an absence of electrodes;

an absence of a magnetic diffusion field consuming a large amount ofenergy;

a high electronic density in the kinds of gases in question (anelectronic density with argon of the order of 10¹² to 10¹³ per cubiccentimeter).

a large pressure operating range (from some 10⁻³ torr to a few torr);

a stability and reproducibility of the operating conditions;

a large cross section and a large volume of the plasma;

an adjustment of the volume of the plasma by confinement of theelectromagnetic field (adjustment of the short circuit means 52);

an adjustment of the volume of the plasma by the value of the incidentpower (adjustment of the power output of the generator 32);

homogeneity of the distribution of the kinds of gases in question;

a very good ultrahigh frequency non-resonant coupling;

a stability of the non-resonant coupling for various gas mixtures.

We claim:
 1. A plasma reactor comprising: a housing (10) capable ofreceiving a selected gaseous flow, a microwave generator (32) and waveguide means (34) for passing the microwaves to the housing (10) along anon-resonant coupling wherein the housing (10) does not dissipate, inthe form of electromagnetic radiation, the ultrahigh frequency energyimparted thereto in the presence of gas, characterized in that, sincethe housing (10) is of a large size, the end portion (38) of the waveguide means is progressively reduced in one direction and widened in theother until it reaches a flat rectangular cross section completelysurrounding the said housing (10) and produces a non-resonant coupling(50) therewith.
 2. A plasma reactor according to claim 1, characterizedin that provision is made for a piston type short circuit means (50),completely surrounded by the end portion (38) of the wave guide meansand disposed on the diametrically opposite side relative to themicrowave intake, said piston (50) being adjusted so as to define adesired ultrahigh frequency electric field in the housing (10).
 3. Aplasma reactor according to claim 2, characterized in that the shortcircuit means (50) is immovable.
 4. A plasma reactor according to claim2, characterized in that the short circuit means (50) is movable.
 5. Aplasma reactor according to claim 2, characterized in that the housing(10) comprises an internal tubular element (12) placed in a vacuum andbeing concentric with the housing (10), the wall of the said tubularelement (12) being constituted by a material having low dielectriclosses, such as quartz.
 6. A plasma reactor according to claims 5,characterized in that provision is made inside the housing (10) for anadditional short circuit means (52) coupled with the tubular element(12) and having first and second annular discs (53, 54) surrounding thetubular element (12), said annular discs (53, 54) being separated fromeach other by a predetermined and adjustable distance with a view toconfining the plasma in the portion of the tubular element (12)comprised between said annular discs (53, 54).
 7. A plasma reactoraccording to claim 6, characterized in that the additional short circuitmeans (52) is immovable.
 8. A plasma reactor according to claim 6,characterized in that the additional short circuit means (52) ismovable.
 9. A plasma reactor according to claim 1, characterized in thatthe wave guide comprises in its intermediate portion (37), adjustmentmeans such as a plurality of adjustable penetrating screws (40).
 10. Aplasma reactor according to claim 1, characterized in that the waveguide comprises in its end portion tapering in one direction and widenedin the other, adjustment means such as a plurality of adjustablepenetrating screws (40).
 11. A plasma reactor intended for a surfacetreatment of a sample according to claim 6, characterized in that thesample (67) is supported on a sample carrier (66) placed inside thetubular element (12) in the portion comprised between the two annulardiscs (53, 54) wherein the plasma is formed.
 12. A plasma reactorintended for a surface treatment of a sample according to claim 6,characterized in that the sample (67) is supported on a sample carrier(66) placed into the tubular element (12) and outside the portioncomprised between the two annular discs (53 and 54) wherein the plasmais formed.
 13. A plasma reactor according to claim 11, characterized inthat it comprises moreover means (80) for generating an electromagneticradiofrequency field between means delivering the gaseous flow and thesample carrier (66) with a view to polarizing the sample (67).
 14. Aplasma reactor according to claim 11, characterized in that itcomprises, moreover, heating means for heating the sample.
 15. A plasmareactor according to claim 11, characterized in that provision is madefor a lock chamber for the insertion of the samples to allow the sample(67) to be placed on the sample carrier (66).